Method of purifying molten silica

ABSTRACT

An improved quality vitreous silica boby and/or improved quality product made at high temperature in a vitreous silica vessel is/are obtained by applying a polarizing potential across the boundary surfaces of the vitreous silica body or vessel to cause migration of impurity ions away from one of the boundary surfaces thereof. Single crystal silicon (10) of reduced alkali content is drawn from melt (12) in a vitreous silica crucible (14) with a polarizing voltage applied across the wall of the crucible.

This invention relates to a method of improving the quality of a productmade from or made in contact with a body of vitreous silica.

In one aspect, the invention relates to a method of making an improvedvitreous silica product, such as a crucible, a tube or a plate, having areduced impurity content, and to a product produced by the method.

The invention also relates to an improved method for using a vitreoussilica vessel for the high temperature processing of a materialcontained therein. Particularly valuable uses of this method of theinvention are found in the treatment of molten semiconductor materialsin a vitreous silica crucible (e.g. in the drawing of a single crystalof silicon).

DISCUSSION OF PRIOR ART

An arc moulded crucible (AMC) is normally produced by fusing a powdermaterial under the influence of an electrical arc while the powdermaterial (e.g. quartz powder) is held in place in a rotating mould (e.g.of water cooled metal) by centrifugal force, with or without theapplication of a vacuum via the mould wall(s).

A vitreous silica crucible is commonly used to contain a melt from whicha single crystal is drawn. In the case of semiconductor materials, highpurity is of vital importance and much effort has been, and is stillbeing, given to avoiding impurity contamination of the melt during thecrystal pulling operation. The crucible is one potential source of suchcontamination.

In the case of the pulling of a silicon crystal from melt in a vitreoussilica crucible, alkali impurities may transfer to the molten siliconfrom the wall(s) of the crucible and a variety of different, oftentime-consuming and otherwise expensive, procedures have been proposedfor the purification of the starting material used for a vitreous silicacrucible to reduce the alkali impurities therein.

It has now been discovered that a reduced impurity content in materialfused in a vitreous silica crucible can be obtained by the simpleexpedient of applying a polarising potential across the wall(s) of thecrucible, at least while the latter is at a temperature in excess of700° C., for such a period and with such a polarity that ions of the oreach impurity will migrate across the wall(s) of the crucible away fromthe inside surface thereof.

Impurity migration can be effected when the crucible is first fused fromthe starting material and/or by applying a correctly polarised potentialacross the wall(s) of the crucible when it contains melt. In the case ofalkali impurity ions (which so far appear to be the impurities mosteasily removed from the crucible wall/melt interface) a voltage withpositive polarity on the inside and negative polarity on the outside ofthe crucible will be required. Voltages between 1 and 2000 volts appearto be effective over the temperatures and times typical for pulling asingle crystal from a bath of molten semiconductor material. Themigration rate is a function of temperature and applied voltage andpreferably the temperature of the inside surface of the crucible wall(s)is in excess of 900° C.

The crucible wall(s) is/are not a significant barrier to the diffusionof impurities (e.g. Na, K or Li) which in the absence of anelectrolysing potential may migrate through the wall(s) from thecrucible holder, and which in the presence of an electrolysing potentialcan migrate into the wall(s) from melt in the crucible.

Although the invention is thought to have an important commercial impactin the areas of the high temperature production of products fromvitreous silica vessels and the manufacture of such vessels with reducedalkali impurity content, it will be appreciated that vitreous silicatubing whose impurity content has been reduced by electrolysis can haveother useful applications than as an intermediary in the manufacture ofa vitreous silica vessel.

Hence it should be appreciated that the invention also extends to theproduction of improved vitreous silica products (e.g. tubing) either bya one-stage process in which an ion-migrating potential is appliedacross the product during manufacture, or by a two-stage process inwhich an ion-migrating potential is applied across a heated billet priorto further processing (e.g. drawing) with or without furtherelectrolysis during that further processing.

Once ion migration in a vitreous silica body has been achieved using theprinciples described herein, it would often be desirable to remove anyion-enhanced region from the body so that even if back diffusion shouldsubsequently occur, there will be a net improvement in impurity contentof the vitreous silica body.

SUMMARY OF THE INVENTION

Thus, in its broadest aspect, the present invention relates to a methodof improving the quality of a product made from, or made in contactwith, a body of vitreous silica, which is characterised in that impurityions are made to migrate away from one boundary surface of the bodytowards an opposite boundary surface thereof by applying a polarisingpotential across the boundary surfaces of the body while the body ismaintained at a temperature above 700° C.

The polarising potential is typically poled to make alkali metal ionsmigrate away from the said one boundary surface. Typical temperaturesare in the range 800° to 2000° C., typical process times are from one toa few tens of hours at the lower end of the range and a few minutes toan hour at higher temperatures and typical polarising potentials from afew tens of volts to a few kilovolts.

The method can be used to make tubing and other vitreous silica productsof improved purity.

It is thus feasible to electrolyse tubing during fusing to causemigration of ions away from one wall surface and then to produce thedesired product either from a powder starting material taken fromion-depleted regions of the tubing (e.g. by grinding off the ion-richregion(s)), or directly from the electrolysed tubing using ion-depletedregions for the inside walls. Preferably polarising potential should bereapplied every time the wall material is heated above about 750° C. andcertainly above 900° C. if some back-diffusion is not to occur but if asufficient impurity depletion has been achieved during manufacture ofthe product or its starting material, such back-diffusion may notmatter. In practice, however, since electrolysing a vitreous silicavessel during use is neither difficult nor expensive to achieve, itcould be employed even where a low-alkali-impurity content vitreoussilica material was used in its manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be more fully described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic indication of the invention applied to improvesingle crystal semiconductor material drawn from a vitreous silicacrucible and as described in the following Examples 10 to 12,

FIG. 2 shows a vitreous silica crucible whose purity has been improvedby the method of the invention which is being processed as described inthe following Examples 1 and 2,

FIG. 3 shows how a vitreous silica crucible can be made in accordancewith the invention in the manner described in the following Examples 3and 4,

FIGS. 4 and 5 show a crucible being made in the manner described,respectively, in the following Examples 5 to 7 and Example 8, and

FIG. 6 shows how the quality of vitreous silica tubing can be improvedas described in the following

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows, purely schematically, a single crystal 10 of silicon beingdrawn from a bath 12 of molten silicon contained in a vitreous silicaarc moulded crucible 14. In the usual way, the crucible 14 is containedwithin a graphite susceptor 16 heated by an induction coil (not shown)and the crystal 10 depends from a seed crystal 10a which has been slowlydrawn upwards out of the surface of the bath 12.

A d.c. potential is applied between the seed crystal 10a and thesusceptor 16 with the polarity as shown, this potential being maintainedthroughout the pulling operation. The magnitude of the applied potentialcan vary from a minimum which is sufficient to overcome contactresistances and ionisation potentials and establish a current of a fewmicroamps, to a maximum where the high voltage causes arcing or otherproblems. In practice, a potential of between a few volts and a fewhundred volts would normally be used. FIG. 1 is further discussed in thefollowing Examples 10 to 13.

EXAMPLES OF THE INVENTION Example 1

(see FIG. 2)

A previously manufactured crucible 20 of 300 mm diameter with a 6 mmwall thickness was heated to 1050° C. in nitrogen. An internalelectrodes 21, consisting of high purity carbon powder loosely filledinto the crucible 20, and an external electrode was provided by agraphite holder 22 into which the crucible 20 fitted loosely. The gapbetween the holder 22 and the crucible 20 was filled with high puritycarbon powder 23. Care was taken to ensure excess carbon powder did notcause short circuits. The upper 10 mm of the crucible protruded abovethe holder and internal powder fill to act as a barrier to surfacetracking by the applied voltage. The holder was made the negativeelectrode.

The voltage was applied gradually on the crucible reaching 1050° C. tokeep the electrolysing current below 50 mA. After 40 minutes, the fullvoltage of 2.5 KV could be applied.

Electrolysis continued for 4 hours, then the temperature was allowed todrop to room temperature with the full voltage still applied.

Analysis of the crucible material after this treatment is shown in Table1 as AMC 3.

Example 2

A second crucible was treated as in Example 1 except that the polarisingvoltage was switched off at 800° C. as the crucible was coolingfollowing electrolysis at 1050° C.

Analysis of the crucible is shown in Table 1 as AMC 4. From the resultsit can be seen that back diffusion of the alkali ions is insignificantbelow 800° C.

Example 3

(see FIG. 3)

A further crucible 30 was heated directly in a spinning graphite mould31 using an oxy propane flame 32 so that the crucible softened and cameinto intimate contact with the mould. The mould was made negative andthe burner positive using a voltage of 3.8 KV. The high electricalimpedance of the flame greatly reduced the voltage available forelectrolysis but some improvement was measured as can be seen from thefigures shown in Table 1 as AMC 10. The time of electrolysis was 5minutes.

Example 4

A further crucible was treated as in Example 3 except that an R.F.induction plasma replaced the flame 32. The analysis of this treatedcrucible is shown as AMC 14 in Table 1.

Example 5

(see FIG. 4)

A crucible 40 was manufactured using the spinning mould method to holdhigh purity quartz powder in position. Heating was with an arc 41 andthe mould 42 was of water-cooled metal. An electrolysing voltage of 10KV was applied for the final 2 minutes of heating using the arc 41 asthe positive electrode and the mould 42 as the negative electrode. Theanalytical result is shown as AMC 21 in Table 1.

Example 6

A crucible was manufactured as for Example 5 except that an uncooledgraphite mould was used in place of the water-cooled mould 42. Theanalytical result is shown in Table 1 as AMC 23. The improved resultwhen compared with Example 5 is believed to be due to the higherelectrical resistance of the quartz powder kept cold by the water-cooledmould in Example 5, reducing the voltage available for electrolysis.

Example 7

A crucible was manufactured as for Example 6 except that during fusion apartial vacuum of 8-7 kPa was applied between the mould 42 and theforming crucible 40 via a pipe 45.

The analytical result is shown as AMC 31. The lower impurity content inthis case is believed to be due to partial ionisation of the gas in thegap between the mould and the forming crucible due to the partial vacuumand this ionised gas acting as the negative electrode.

                  TABLE 1                                                         ______________________________________                                                         Noted impurity in ppm                                                         by weight                                                    Crucible                                                                              Position Surface   Na.sub.2 O                                                                           K.sub.2 O                                                                           Li.sub.2 O                            ______________________________________                                        AMC3    Side     Inner     <0.1   <0.1  <0.1                                          Wall     Outer     1.5    4.2   3.3                                           Base     Inner     0.2    0.2   0.2                                           Wall     Outer     1.6    3.3   2.8                                   AMC4    Side     Inner     <0.1   <0.1  <0.1                                          Wall     Outer     2.5    2.0   4.4                                           Base     Inner     <0.1   <0.1  <0.1                                          Wall     Outer     0.1    0.2   0.3                                   AMC10   Mean     Inner     0.9    1.5   0.2                                           Value    Outer     2.2    2.0   2.0                                   AMC14   Mean     Inner     <0.1   <0.1  <0.1                                          Value    Outer     0.2    0.4   0.2                                   AMC21   Mean     Inner     0.9    1.2   0.2                                           Value    Outer     1.4    2.5   2.0                                   AMC23   Mean     Inner     0.2    1.3   <0.1                                          Value    Outer     0.9    2.5   0.5                                   AMC31   Mean     Inner     <0.1   0.2   <0.1                                          Value    Outer     0.1    0.4   0.1                                   AMC33   Mean     Inner     <0.1   <0.1  <0.1                                          Value    Outer     0.2    0.5   <0.1                                  Starting Material for AMC 3, 4,                                                                  5.6      5.4     3.8                                       10, 14                                                                        Starting Material for AMC 21, 23,                                                                1.4      2.5     3.2                                       31                                                                            ______________________________________                                    

Example 8

(see FIG. 5)

A manufactured crucible 50 was placed over a closely fitting graphiteinternal mould 51 and heated externally with an oxy-propane ribbonburner 52. The temperature reached on the surface of the crucible wassufficient to remelt it.

A potential difference was applied between the burner 52 and the mould51 of 4.5 KV. The mould was rotated at 1 RPM so that the flame sweptover all the crucible. The mould was the positive electrode. During theprocessing it was noted that the flame was coloured by the ions beingelectrolysed from the crucible.

An additional result of the heating was that the outside of the cruciblebecame glazed.

The analytical results appear in Table 1 as AMC 33.

Example 9

(see FIG. 6)

A cylindrical pipe 60 of fused quartz with an external diameter of 200mm, a length of 1500 mm and a wall thickness of 25 mm was subject toelectrolysis across the wall with a potential difference of 10 KV.

An inner electrode 61 (anode) was made by coating the inner surface witha layer approximately 2 mm thick consisting of a paste of low alkalititanium dioxide (British Titan Products Ltd.--Grade A-HR) and aproprietary low alkali silica sol (Nalfloc Ltd.--"Nalcoag 1034A").Connection to this electrode was made with a nickel chromium alloy band62 which was a spring fit in the bore.

An outer electrode (cathode) consisted of a layer 63 approximately 2 mmthick coating the whole outer surface except the ends and consisting ofa paste of ferric oxide and a silica sol. Connection to this electrodewas made with an open pitch coil 64 of nickel chromium heat resistingwire.

Electrolysis was carried out at 1050° C. On reaching temperature, thevoltage was gradually increased so as to avoid exceeding the currentlimitation of the power supply (100 ma). Maximum voltage of 10 KV wasreached after 8 hours 40 minutes.

Electrolysis continued for 30 hours, when the furnace was allowed tocool naturally. The voltage was switched off when the pipe had cooled to800° C.

The results of the electrolysis are shown as B4 in Table 2.

Example 10

The cylindrical pipe of fused quartz from Example 9 was machinedexternally to remove 1 mm from the bore and 3 mm from the externalsurface leaving a wall thickness of 21 mm. After cleaning with detergentand dilute hydrofluoric acid it was reheated in a graphite resistancefurnace and drawn into tubing. Some of this tubing was reworked withflames on a glass working lathe to the form of a crucible. The analysisof the crucible is shown in Table 2 as Cl.

                  TABLE 2                                                         ______________________________________                                                   Impurity concentration in ppm                                                 by weight                                                          Surface      Na.sub.2 O  K.sub.2 O                                                                              Li.sub.2 O                                  ______________________________________                                        B4     Inner     <0.1        <0.1   <0.1                                             Centre    <0.1        <0.1   <0.1                                             Outer     13.9        4.8    8.0                                       Tube from B4 0.1         0.1      <0.1                                        C1     Inner     <0.1        <0.1   <0.1                                             Outer     <0.1        0.1    <0.1                                      Starting Material                                                                          8.8         4.3      4.4                                         for B4                                                                        ______________________________________                                    

Example 11

(see FIG. 1)

A single crystal "puller" was modified to allow a voltage to be appliedbetween the silicon single crystal 10a of FIG. 1 and the graphitesusceptor 16 during that growing operation which was carried out inArgon at 1 atmosphere gauge. After the crystal 10 had achieved thedesired diameter, a voltage of 50-1000 V was applied between the crystal(positive) and the susceptor (negative). The voltage was derived from acurrent limited source of 0.010A.

Results for the silicon crystal 10 are shown in Table 3.

Example 12

A single crystal was grown as for Example 11 except that asub-atmospheric pressure of 15-20 torr was used in the puller. Themaximum polarising voltage was limited to 200 V.

The results are shown in Table 3 (S2).

Example 13

A single crystal was grown as for Example 11 except that a coating ofglassy carbon (obtained by the pyrolysis of propane diluted with Argon)had been made on the outside of the quartz crucible before use in orderto improve the electrical contact between the crucible and thesusceptor.

The results are shown in Table 3 (S3).

                  TABLE 3                                                         ______________________________________                                        Resistivity of Single Crystal                                                 ohm-cm P.Type Silicon                                                         ______________________________________                                        Without polarizing Crystal Shoulder 145                                       Voltage            Crystal end 115                                            With polarizing                                                                            S1        Crystal Shoulder 500                                   Voltage                Crystal end 300                                                     S2        Crystal Shoulder 850                                                          Crystal end 600                                                     S3        Crystal Shoulder 1250                                                         Crystal end 1050                                       ______________________________________                                    

From the foregoing Examples, it will be appreciated that theelectrolysing temperature and time conditions are related one to theother and to the wall thickness across which the polarising potential isapplied. In summary, these process conditions are preferably that thebody is maintained for a time of at least 1 hour/mm wall thickness inthe temperature range 800°-1200° C. and at least 1 min/mm thickness inthe temperature range 1201°-2000° C. The effective polarising potentialapplied across the boundary surfaces preferably exceeds 10 V/mmthickness but not 1 KV/mm thickness.

I claim:
 1. A method of improving the purity of melt contacting an innerwall of a vitreous silica containing vessel, wherein impurity ions inthe melt are made to migrate into said inner wall and from the innerwall towards an outer wall of the vessel, by applying a polarisingpotential across said walls, the polarising potential being applied viathe melt in a direction to cause impurity ions in the vitreous silica tomigrate away from said inner wall while the vessel is maintained at atemperature above 1000° C.
 2. The method of claim 1, wherein the melt isa semiconductor material and the polarising potential is applied theretovia a single crystal of the semiconductor material in contact with themelt.
 3. The method of claim 1, wherein the vitreous silica vessel hasits outer surface in electrical contact with carbon and the polarisingpotential is applied across the wall of the vessel via the melt and thecarbon.
 4. The method of claim 1, wherein the polarising potential liesin the range 1 to 2000 volts and the melt is at positive potentialrelative to the outer wall of the vessel.